Forward magnetic models across the southern Apennines: implications for the basement setting

We discuss here the forward magnetic modeling, along a southern
Apennines transect, of the new magnetic map of Italy (CHIAPPINI
et alii, 2000a). This map has been realized through an integrated
digital magnetic anomaly database built for the Italian territory and
the surrounding sea. For the first time, all the magnetic observations
made onshore and offshore the Italian peninsula have been compiled
into a new digital database, that gives a complete regional scale
view of the magnetic anomaly crustal field over the area. The new
magnetic map shows a low-amplitude (~20 nT), long-wavelength
positive anomaly along the external southern Apennines, and negative
anomalies over both the Tyrrhenian margin of southern Italy
and the Apulian foreland. South of the Vulture volcano (~41° N), the
positive magnetic anomaly is elongated and exactly parallel to the
external belt. Furthermore, the eastern anomaly margin is exactly
parallel to the belt front, and located few tens of kilometers southwest
of it. The negative pattern of the Tyrrhenian Sea and margins is
likely due to the high thermal flow, which significantly reduces the
thickness of the susceptive crustal layer. A magnetic modeling was
performed along a selected southern Apennines transect (located ca.
50 km south of the CROP04 profile), where the shallow crustal setting
(down to 5-6 km depth) is well-known by seismic and drill-hole
data. Seismic data and oil wells suggest that the top of the magnetic
basement cannot be shallower in the belt than in the Apulian foreland,
where it was reached at ~6 km depth by the Puglia 1 well.
Therefore, the positive signature of the external southern Apennines
requires a strongly magnetic body beneath the belt. This source
must be definitely more susceptive than the basement of the Apulian
foreland. Since the solution of magnetic modeling is not unique, we
show here three different settings that are able to bring into coincidence
the observed and modeled residuals. First, we speculate that,
within the belt, the basement located beneath the Apulian carbonates
(Apulian basement) is differentiated and contaminated at depth
by susceptive magmas or mantle fluids. Then, we explore the possibility
that a very magnetic composite internal crustal wedge is tectonically
interposed between the Apulian carbonates and basement.
Two possible settings of such composite wedge are modeled and
discussed. The uniformity of the magnetic anomaly pattern along
250 km of the belt, the remarkable parallelism between the southern
Apennine positive residuals and the superficial belt front, associated
with the gravity data, the estimates of Moho depth, and the flexural
behavior of the Apulian lithosphere beneath the belt, support the
two latter models.

AGIP SPA. ITALIA (1981) - Carta Magnetica. Anomalie del Campo
magnetico residuo, scale 1:500,000, S. Donato Milanese (Italy).
ARISI ROTA F. & FICHERA R. (1987) - Magnetic interpretation related
to geo-magnetic provinces: the Italian case history. Tectonophysics,
138, 179-196.
CARATORI TONTINI F., FAGGIONI O., GIORI I. & STEFANELLI P. (2003)
- La nuova carta aeromagnetica d’Italia: confronto con i principali
lineamenti tettonico-strutturali, Abstract presented at the 22°
GNGTS Congress, Roma (Italy), October 18-20, 2003.
CASERO P., ROURE F., ENDIGNOUX L., MORETTI I., MULLER C., SAGE
L. & VIALLY R. (1988) - Neogene geodynamic evolution of the
southern Apennines. Mem. Soc. Geol. It., 41, 109-120.
CATALDI R., MONGELLI F., SQUARCI P., TAFFI L., ZITO G. & CALORE
C. (1995) - Geothermal ranking of Italian territory. Geothermics,
24 (1), 115-129.
CHIAPPINI M. & SPERANZA F. (2002) - The new magnetic map of Italy
at sea level: implications for the deep structural style of the Apennine
belt. Boll. Soc. Geol. It., Vol. Spec. n. 1, 13-23.
CHIAPPINI M., MELONI A., BOSCHI E., FAGGIONI O., BEVERINI N.,
CARMISCIANO C. & MARSON I. (2000a) - Onshore- offshore integrated
shaded relief magnetic anomaly map at sea level of Italy
and surrounding areas, scale 1:1,500,000. In Annali di Geofisica,
43 (5), Plate 1.
CHIAPPINI M., MELONI A., BOSCHI E., FAGGIONI O., BEVERINI N.,
CARMISCIANO C. & MARSON I. (2000b) - Shaded Relief Total Field
Magnetic Anomaly Map of Italy and surrounding marine areas at
sea level. Annali di Geofisica, 43 (5), 983-989.
CHIARABBA C. & AMATO A. (1997) - Upper-crustal structure of the Benevento
area (southern Italy): fault heterogeneities and potential
for large earthquakes. Geophys. J. Int., 130, 229-239.
CONSIGLIO NAZIONALE DELLE RICERCHE (1991) - Structural model of
Italy, scale 1:500,000, and gravity map. Progetto Finalizzato Geodinamica,
Roma.
DEL BEN A., FINETTI I., MONGELLI F. & ZITO G. (1994) - Seismic and
heat flow study of the southern Adriatic sea. Boll. Geof. Teor.
Appl., 36, 29-44.
IMPROTA L., IANNACCONE G., CAPUANO P., ZOLLO A. & SCANDONE P.
(2000) - Inferences on the upper crustal structure of the Southern
Apennines (Italy) from seismic refraction investigations and subsurface
data. Tectonophys, 317, 273-297.
MALINVERNO A. & RYAN W.B.F. (1986) - Extension in the Tyrrhenian
Sea and shortening in the Apennines as result of arc migration
driven by sinking of the lithosphere, Tectonics, 5 (2), 227-245.
MATTEI M., SAGNOTTI L., FACCENNA C. & FUNICIELLO R. (1997) -
Magnetic fabric of weakly deformed clayey sediments in the Italian
peninsula: Relationships with compressional and extensional
tectonics. Tectonophysics, 271, 107-122.
MAZZOLI S., CORRADO S., DE DONATIS M., SCROCCA D., BUTLER R.,
DI BUCCI D., NASO G., NICOLAI C. & ZUCCONI V. (2000) -Time
and space variability of «thin-skinned» and «thick-skinned» thrust
tectonics in the Apennines (Italy). Rend. Acc. Naz. dei Lincei,
11 (1), 5-39.
MENARDI NOGUERA A. & REA G. (2000) - Deep structure of the Campanian-
Lucanian Arc (Southern Apennine, Italy). Tectonophysics,
324, 239-265.
MOLINA F. & DE SANTIS A. (1987) - Considerations and proposal for a
best utilization of IGRF over areas including a geomagnetic observatory.
Phys. of the Earth and Planet. Int., 48, 379-385.
MOLINA F., ARMANDO E., BALIA R., BATTELLI O., BOZZO E., BUDETTA
G., CANEVA G., CIMINALE M., DE FLORENTIIS N., DE SANTIS A.,
DONNALOIA M., ELENA A., ILICETO V., LANZA R., LODDO M., MELONI
A., PINNA E., SANTARATO G. & ZAMBRANO R. (1986) - Geomagnetic
Survey of Italy. Repeat station network and magnetic maps.
Progetto Finalizzato Geodinamica, Pubblicazione ING, 554.
MOSTARDINI F. & MERLINI S. (1986) - Appennino centro meridionale,
sezioni geologiche e proposta di modello strutturale. Mem. Soc.
Geol. It., 35, 177-202.
NICOLICH R. (1981) - Crustal structures in the Italian peninsula and
surrounding areas: a review of DSS data. In F.C. Wezel (Ed.), Sedimentary
basins of Mediterranean margins, CNR Italian
project of Oceanography, Tecnoprint, Bologna (Italy).
OGNIBEN L. (1969) - Schema introduttivo alla geologia del confine
calabro-lucano. Mem. Soc. Geol. It., 8, 453-763.
PATACCA E. & SCANDONE P. (2001) - Geological interpretation of
the CROP 04 seismic line. Sottoprogetto CROP 04 - Appennino
meridionale, Riunione di lavoro, Pisa, december 20-21, 2001
(Abstract).
PATACCA E., SARTORI R. & SCANDONE P. (1990) - Tyrrhenian Basin
and Apenninic Arcs: kinematic relations since late Tortonian times.
Mem. Soc. Geol. It., 45, 425-451.
ROURE F., CASERO P. & VIALLY R. (1991) - Growth processes and melange
formation in the southern Apennines accretionary wedge.
Earth and Planet. Sci. Lett., 102, 395-412.
ROYDEN L.E. (1988) - Flexural behavior of the continental lithosphere
in Italy: Constraints imposed by gravity and deflection data. J.
Geophys. Res., 93, 7747-7766.
ROYDEN L.E., PATACCA E. & SCANDONE P. (1987) - Segmentation and
configuration of subducted lithosphere in Italy: an important control
on thrust-belt and foredeep-basin evolution. Geology, 15, 714-717.
SCARASCIA S., LOZEJ A. & CASSINIS R. (1994) - Crustal structures of
the Ligurian, Tyrrhenian and Ionian seas and adjacent onshore
areas interpreted from wide-angle seismic profiles. Boll. Geof.
Teor. Appl., 36, 5-19.
SPERANZA F. & CHIAPPINI M. (2002) - Thick-skinned tectonics in
the external Apennines, Italy: New evidence from magnetic anomaly
analysis. J. Geophys. Res., 107 (B11), 2290, doi:
10.1029/2000JB000027.